This invention relates to radially stratified flame core burners, which are employed in the firing systems of fossil fuel-fired furnaces, and more specifically, to a method for effecting control over a radially stratified flame core burner.
Fossil fuels have been successfully burned in furnaces for a long time. Recently though, more and more emphasis has been placed on the minimization as much as possible of air pollution. In this connection, with reference in particular to the matter of NO.sub.X control it is known that during the combustion of fossil fuels in furnaces oxides of nitrogen are created. Moreover, it is also known that these oxides of nitrogen are created primarily by two separate mechanisms, which have been identified to be thermal NO.sub.X and fuel NO.sub.X.
Continuing, thermal NO.sub.X results from the thermal fixation of molecular nitrogen and oxygen in the air that is employed in the course of effecting the combustion of the fossil fuel. The rate of formation of thermal NO.sub.X is extremely sensitive to local flame temperature and somewhat less so to local concentration of oxygen. Virtually all thermal NO.sub.X is formed in the region of the flame that is at the highest temperature. The thermal NO.sub.X concentration is subsequently "frozen" at the level prevailing in the high temperature region by the thermal quenching of the combustion gases. The flue gas thermal NO.sub.X concentrations are, therefore, between the equilibrium level characteristic of the peak flame temperature and the equilibrium level at the flue gas temperature.
On the other hand, fuel NO.sub.X derives from the oxidation of organically bound nitrogen in certain fossil fuels such as coal and heavy oil. The formation rate of fuel NO.sub.X is strongly affected by the rate of mixing of the fossil fuel and air stream in general, and by the local oxygen concentration in particular. However, the flue gas NO.sub.X concentration due to fuel nitrogen is typically only a fraction, e.g., 20 to 60 percent, of the level which would result from complete oxidation of all nitrogen in the fossil fuel. Thus, it should now be readily apparent from the preceding that overall NO.sub.X formation is a function both of local oxygen levels and of peak flame temperatures.
Over the years, there have been different approaches pursued in the prior art insofar as concerns addressing the need to limit emissions of the NO.sub.X that is created as a consequence of the combustion of fossil fuels in furnaces. The focus of one such approach has been on developing so-called low NO.sub.X firing systems suitable for employment in fossil fuel-fired furnaces. By way of exemplification and not limitation in this regard, one example of such a low NO.sub.X firing system is that which forms the subject matter of U.S. Pat. No. 5,020,454 entitled "Clustered Concentric Tangential Firing System," which issued on Jun. 4, 1991 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,020,454, a clustered concentric tangential firing system is provided that includes a windbox, a first cluster of fuel nozzles mounted in the windbox and operative for injecting clustered fuel into the furnace so as to create a first fuel-rich zone therewithin, a second cluster of fuel nozzles mounted in the windbox and operative for injecting clustered fuel into the furnace so as to create a second fuel-rich zone therewithin, an offset air nozzle mounted in the windbox and operative for injecting offset air into the furnace such that the offset air is directed away from the clustered fuel injected into the furnace and towards the walls of the furnace, a close coupled overfire air nozzle mounted in the windbox and operative for injecting close coupled overfire air into the furnace, and a separated overfire air nozzle mounted in the windbox and operative for injecting separated overfire air into the furnace.
Another example of such a low NO.sub.X firing system is that which forms the subject matter of U.S. Pat. No. 5,315,939 entitled "Integrated Low NO.sub.X Tangential Firing System," which issued on May 31, 1994 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,315,939, an integrated low NO.sub.X tangential firing system is provided that includes pulverized solid fuel supply means, flame attachment pulverized solid fuel nozzle tips, concentric firing nozzles, close-coupled overfire air, and multi-staged separate overfire air and when employed with a pulverized solid fuel-fired furnace is capable of limiting NO.sub.X emissions therefrom to less than 0.15 lb./106 BTU while yet maintaining carbon-in-flyash to less than 5% and CO emissions to less than 50 ppm.
Yet another example of such a low NO.sub.X firing system is that which forms the subject matter of U.S. Pat. No. 5,343,820 entitled "Advance Overfire Air System for NO.sub.X Control," which issued on Sep. 6, 1994 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,343,820, an advanced overfire air system for NO.sub.X control is provided that includes multi-elevations of overfire air compartments to which overfire air is supplied such that there is a predetermined most favorable distribution of overfire air therebetween, such that the overfire air exiting from the separated overfire air compartments establishes a horizontal "spray" or "fan" distribution of overfire air exiting from the separated overfire air compartments at velocities significantly higher than the velocities employed heretofore.
The focus of another approach, which has been pursued in the prior art to address the need to limit emissions of the NO.sub.X that is created as a consequence of the combustion of fossil fuels in furnaces, has been on developing so-called low NO.sub.X burners that are suitable for integration into the firing systems that are employed in fossil fuel-fired furnaces. By way of exemplification and not limitation in this regard, one example of such a low NO.sub.X burner is that which forms the subject matter of U.S. Pat. No. 4,422,931 entitled "Method Of Combustion Of Pulverized Coal By Pulverized Coal Burner," which issued on Dec. 27, 1983 and which on its face is assigned to Kawasaki Jukogyo Kabushiki Kaisha of Kobe, Japan. In accordance with the teachings of U.S. Pat. No. 4,422,931, a low NO.sub.X burner is provided wherein pulverized coal is supplied together with primary air through a combustion air outlet of the low NO.sub.X burner and caused by a swirler to be injected into the furnace while flowing slowly in vortical form. Secondary air is injected into the furnace with exhaust gas through an inner annular outlet surrounding the combustion air outlet, the secondary air either flowing slowly in vortical form or not flowing in vortical form as the case may be. Tertiary air is injected into the furnace with exhaust gas through an outer annular outlet surrounding the inner annular outlet while flowing in vortical form. Pulverized coal supplied to the furnace together with primary air is combusted to form a primary flame. The primary flame is formed by slow combustion of the pulverized coal at low temperature with low O.sub.2 and is low in brightness, because the primary air is about 20-30% in amount of the air necessary for combusting all the pulverized coal supplied therewith to the furnace and mixing of secondary and tertiary air therewith is prohibited. Combustion of a volatile component of the pulverized coal is mainly responsible for formation of the primary flame, so that the pulverized coal is combusted slowly at low temperature with a flame of low brightness. In this type of combustion, production of NO.sub.X is greatly produced and the non-combusted components, such as hydrocarbons which are activated intermediate products responsible for denitration reaction, NH.sub.3, HCN and CO, are produced in large amounts and exist for a prolonged period of time in non-combusted condition. Thus, these non-combusted components react with NO.sub.X to N.sub.2. Char which is produced in large amounts as a non-combusted component of the primary flame is combusted in the secondary flame. The residual volatile component is combusted mainly by the secondary air ejected through the inner annular outlets to form a secondary flame. Most of the char is combusted by the secondary air and the tertiary air to form a tertiary flame range. The secondary flame and the tertiary flame are formed by the combustion of relatively low speed and low temperature with low O.sub.2, because the secondary and tertiary air is about 55-80% in amount of the air necessary for the combustion of all the pulverized coal and the air contains exhaust gas in 35-60%.
Another example of such a low NO.sub.X burner is that which forms the subject matter of U.S. Pat. No. 4,545,307 entitled "Apparatus For Coal Combustion," which issued on Oct. 8, 1985 and which on its face is assigned to Babcock-Hitachi Kabushiki Kaisha of Tokyo, Japan. In accordance with the teachings of U.S. Pat. No. 4,545,307, a low NO.sub.X burner is provided that includes a pulverized coal pipe inserted into a burner throat on the lateral wall of a combustion furnace and for feeding the coal and air into the furnace, a means for feeding the coal and air into the coal pipe, a secondary air passageway formed between the coal pipe and a secondary air-feeding pipe provided on the outer peripheral side of the coal pipe, a tertiary air passageway formed on the outer peripheral side of the secondary air-feeding pipe, a means for feeding air or an oxygen-containing gas into the secondary air passageway and into the tertiary air passageway, and a bluff body having a cross-section of a L- letter form provided at the tip of the coal pipe.
Still another example of such a low NO.sub.X burner is that which forms the subject matter of U.S. Pat. No. 4,539,918 entitled "Multiannular Swirl Combustor Providing Particulate Separation," which issued on Sep. 10, 1985 and which on its face is assigned to Westinghouse Electric Corp. In accordance with the teachings of U.S. Pat. No. 4,539,918, a low NO.sub.X burner is provided that includes a plurality of tubular members having differing axial lengths and disposed to form a burner basket of sufficient size and axial length to contain axially spaced rich and lean combustion zones, means for supporting the tubular members substantially coaxially and telescopically relative to each other to provide a generally annular path for inlet pressurized gaseous reactant or pressurized air flowing into the low NO.sub.X burner with predetermined axial velocity between each tubular member and the next radially outwardly disposed tubular member, means for imparting a tangential velocity to gaseous reactant entering the low NO.sub.X burner through each annular flow path with the tangential velocity of at least the flows entering the rich combustion zone increasing with increasing flow radius, nozzle means for supplying fuel to the low NO.sub.X burner in at least one predetermined location, the tubular members having respective axial lengths and being so disposed that the axial location of the tubular member outlet ends generally have increasing radii and respectively are located at successive downstream locations, the tangential velocity imparting means and the radial and axial geometry of at least two of the tubular members being coordinated under operating inlet gas pressure and gas axial velocity conditions to a) define the rich combustion zone in an upstream portion of the low NO.sub.X burner where high temperature oxygen deficient combustion occurs with flame stabilizing recirculation flow and substantially without net NO.sub.X formation and b) produce a toroidal vortex in the rich combustion zone with recirculating combustion air being recuperatively supplied substantially by the swirling inlet annular air flow after it has cooled the inner wall surfaces of the tubular members about the rich combustion zone and c) provide sufficient fuel particulate residence time in the rich combustion zone to permit particulate burning prior to centrifugal separation of particulates toward the low NO.sub.X burner wall surface, the tangential velocity imparting means and the radial and axial geometry of at least two of the tubular members located outwardly from the tubular members about the rich combustion zone being coordinated under operating inlet gas pressure and gas axial velocity conditions to define the lean combustion zone and to produce a toroidal vortex in the lean combustion zone, the tubular members being arranged to provide a throat section into which the rich combustion zone converges and from which the lean combustion zone diverges, and means for collecting and withdrawing from the combustion particulates separated from the flow as it passes through the throat section.
Yet another example of such a low NO.sub.X burner is that which forms the subject matter of U.S. Pat. No. 4,845,940 entitled "Low NO.sub.X
Rich-Lean Combustor Especially Useful In Gas Turbines," which issued on Jul. 11, 1989 and which on its face is assigned to Westinghouse Electric Corp. In accordance with the teachings of U.S. Pat. No. 4,845,940, a low NO.sub.X burner is provided that includes tubular wall means having at least three successive tubular wall portions disposed in successive downstream locations and having respectively increasing dimensions in the radial direction to provide a generally outwardly diverging combustor envelope along the axial direction that defines an outwardly diverging combustion zone for low NO.sub.X combustion, means for supporting the tubular wall portions relative to each other to provide a rigid structure for the low NO.sub.X burner, nozzle means for supplying fuel to the low NO.sub.X burner in at least one predetermined location, each successive pair of adjacent tubular wall portions being structured to define a generally annular inlet flow path extending in the radial direction between the outer surface of the radially inward upstream wall portion of the pair and the inner surface of the radially outward downstream wall portion of the pair and further extending downstream in the axial direction along the inner surface of the radially outward downstream wall portion of the pair so that successive annular flow paths axially overlap to enable the annular flows to combine at least partly for swirling radially inward flow into the combustion zone, the wall portions further being sized and structurally coordinated so that the total annular air flow includes substantially all of the pressurized inlet air flow needed for complete fuel burning in the combustion zone other than any nozzle atomizing air flow or other special air flow that may be provided and such that the combustion air flows inwardly at a rate needed to support rich combustion along the axial region of the combustion zone thereby enabling leaner combustion radially outwardly and axially downstream thereof within the combustion zone, first swirl means for imparting a tangential velocity to inlet air flow through the first and radially inmost annular flow path, second swirl means for imparting a tangential velocity to inlet air flow through the second annular flow path located radially outwardly and axially downstream from the first annular flow path, the first and second swirl means being interrelated to produce a negative radial gradient in the tangential velocities of the inlet air flows through the first and second annular paths, and the tangential velocities decreasing with increasing radius and being operative within the diverging envelope of the combustion zone under operating inlet air pressure and gas axial velocity conditions to produce a depression of the axial velocity on the combustor axis with substantially all of the combustion air being recuperatively supplied by the swirling annular inlet flows after cooling the inner surfaces of the wall portions defining the combustion zone.
Yet a further example of such a low NO.sub.X burner is that which forms the subject matter of U.S. Pat. No. 5,411,394 entitled "Combustion System For Reduction Of Nitrogen Oxides," which issued on May 2, 1995 and which on its face is assigned to Massachusetts Institute of Technology. In accordance with the teachings of U.S. Pat. No. 5,411,394, a low NO.sub.X burner for the combustion of gaseous, liquid and solid fuels is provided, which is characterized by the fact that the fluid dynamic principle of radial stratification by the combustion of swirling flow and a strong radial gradient of the gas density in the transverse direction to the axis of flow rotation is used to damp turbulence near the burner and hence to increase the residence time of the fuel-rich pyrolyzing mixture before mixing with the rest of the combustion air to effect complete combustion.
Notwithstanding the fact that over the years there have been different approaches pursued in the prior art insofar as concerns addressing the need to limit emissions of the NO.sub.X that is created as a consequence of the combustion of fossil fuels in furnaces, a need still exists in the prior art to improve upon what has been accomplished to date in the pursuance of these different approaches. More specifically, low NO.sub.X firing systems constructed in accordance with the teachings of the three issued U.S. patents relating to low NO.sub.X firing systems to which reference has been made hereinbefore have been demonstrated to be operative for the purpose for which they have been designed. Similarly, low NO.sub.X burners constructed in accordance with the teachings of the five issued U.S. patents relating to low NO.sub.X burners to which reference has been made hereinbefore have been demonstrated to be operative for the purpose for which they have been designed.
In particular, although low NO.sub.X burners of the type that forms the subject matter of U.S. Pat. No. 5,411,394, i.e., so-called radially stratified flame core burners, have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless existed a need for further improvements to be made relating to such radially stratified flame core burners. More specifically, there has been evidenced in the prior art a need to be able to effect control over a radially stratified flame core burner. To this end, furnaces in which the combustion of fossil fuels takes place do not all embody the same depth. Thus, although radial stratification can be accomplished so long as the furnace in which the radially stratified flame core burner is being employed embodies a predetermined depth, if the furnace in which it is desired to employ a radially stratified flame core burner, however, embodies a depth other than the aforereferenced predetermined depth, then there exists a need to be able to effect control over the radially stratified flame core burner such that the reduction in NO.sub.X emissions sought to be attained through the use of the radially stratified flame core burner can nevertheless still be realized therewith.
To thus summarize, a need has been evidenced in the prior art for a new and improved method for effecting control over a radially stratified flame core burner such that regardless of the depth that a furnace may embody the radially stratified flame core burner will still be effective in enabling the reduction in NO.sub.X emissions, which is sought to be attained therewith, to be realized. Moreover, not only should it be possible when employing such a new and improved method for effecting control over a radially stratified flame core burner to achieve such a reduction in NO.sub.X emissions regardless of the depth that the furnace embodies, but such a reduction in NO.sub.X emissions should also be attainable while yet at the same time the following benefits, which serve to characterize a radially stratified flame core burner, are still capable of being derived through the use of the radially stratified flame core burner. One such benefit is that a radially stratified flame core burner, which is being controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, is still capable, without the use of overfire air or flue gas recirculation, of reducing NO.sub.X emissions to a level that enables state and federal NO.sub.X limits to be met. A second such benefit is that a radially stratified flame core burner, which is being controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, is capable of achieving NO.sub.X values of less than 0.25 lb./MM BTU while firing No. 6 fuel oil. A third such benefit is that a radially stratified flame core burner, which is being controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, embodies the capability therewith of adjusting the angular momentum thereof and of biasing the airflow thereof. A fourth such benefit is that a radially stratified flame core burner, which is being controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, is characterized by the fact that the operating mechanisms thereof are so positioned as to be protected from heat being radiated from the furnace. A fifth such benefit is that a radially stratified flame core burner, which is being controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, possesses multi-fuel capabilities, i.e., oil, natural gas and coal. A sixth such benefit is that a radially stratified flame core burner, which is controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, is capable of being integrated into virtually any new or existing combustion firing system. A seventh such benefit is that a radially stratified flame core burner, which is controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, is capable of being retrofitted to virtually any boiler design. An eighth such benefit is that a radially stratified flame core burner, which is being controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, possesses a burner heat input rating from 1 MM BTU per hour. A ninth such benefit is that a radially stratified flame core burner, which is being controlled by means of such a new and improved method for effecting control over a radially stratified flame core burner, that permits high-grade materials to be selected for use therein in order to thereby address therewith heat and/or corrosion issues.
It is, therefore, an object of the present invention to provide a new and improved method for effecting control over a radially stratified flame core burner.
It is a further object of the present invention to provide such a new and improved method for effecting control over a radially stratified flame core burner such that regardless of the depth that a furnace may embody the radially stratified flame core burner will still be effective in enabling the reduction in NO.sub.X emissions, which is sought to be attained therewith, to be realized.
It is another object of the present invention to provide such a new and improved method for effecting control over a radially stratified flame core burner wherein the radially stratified core burner is still capable, without the use of overfire air or flue gas recirculation, of reducing NO.sub.X emissions to a level that enables state and federal NO.sub.X limits to be met.
It is still another object of the present invention to provide such a new and improved method for effecting control over a radially stratified flame core burner wherein the radially stratified flame core burner is capable of achieving NO.sub.X values of less than 0.25 lb./MM BTU while firing No. 6 fuel oil.
Another object of the present invention is to provide such a new and improved method for effecting control over a radially stratified flame core burner that embodies the capability of adjusting therewith the angular momentum thereof and of biasing therewith the airflow thereof.
A still another object of the present invention is to provide such a new and improved method for effecting control over a radially stratified flame core burner wherein the radially stratified flame core burner is characterized by the fact that the operating mechanisms thereof are so positioned as to be protected from heat being radiated from the furnace.
A further object of the present invention is to provide such a new and improved method for effecting control over a radially stratified flame core burner wherein the radially stratified flame core burner possesses multi-fuel capabilities, i.e., oil, natural gas and coal.
A still further object of the present invention is to provide such a new and improved method for effecting control over a radially stratified flame core burner wherein the radially stratified flame core burner is capable of being integrated into virtually any new or existing combustion firing system.
Yet an object of the present invention is to provide such a new and improved method for effecting control over a radially stratified flame core burner wherein the radially stratified flame core burner is capable of being retrofitted to virtually any boiler design.
Yet a further object of the present invention is to provide such a new and improved method for effecting control over a radially stratified flame core burner wherein the radially stratified flame core burner possesses a burner heat input rating from 1 MM BTU per hour.
Yet another object of the present invention is to provide such a new and improved method for effecting control over a radially stratified flame core burner wherein the radially stratified flame core burner permits high-grade materials to be selected for use therein in order to thereby address therewith heat and/or corrosion issues.